Satellite communications at the present time use conventional channels for which the frequency bands have typical widths less than 60 MHz. New satellite transponders make it possible to have high bit rate channels with wide bandwidths, typically 500 MHz in the Ka band. To use such a bandwidth, there is a prior-art solution, which consists of frequency multiplexing several conventional channels, for example 8. That said, this solution, because of the nonlinearity of the power units, involves intermodulation. To avoid such intermodulation, the sending power of the transponder is reduced. Such a reduction lowers the signal-to-noise ratio on reception and therefore the bit rate.
To avoid this intermodulation, while retaining a maximum sending power, it is proposed, according to another known solution, to time-division multiplex the conventional channels which will then be transmitted in a single high bit rate channel. This high bit rate channel has a higher symbol frequency and also occupies a greater bandwidth. Thus, it is possible, with conventional 60 MHz channels, to occupy a greater bandwidth, for example 500 MHz.
To produce the time-division multiplexing of the conventional channels, one solution provides for the use of symbol frames called PLFRAME (Physical Layer Frames) standardised by the DVB-S2 standard. These are consecutive symbol frames protected by an error-correcting code. These frames all have the same symbol frequency (the inverse of the duration of a symbol) but are sufficiently independent for each to have a different length, modulation and protection rate. The composition of each of the frames is indicated by a header or 90 symbols modulated by a QPSK modulation.
To demodulate a high bit rate channel whose symbol frequency is very high, such as that resulting from the above mentioned time-division multiplexing, one known solution consists of performing the demodulation calculations in parallel. The useful frames are then selected by demultiplexing after this demodulation. However, this solution is not optimal, because it amounts to using computational power on data that will not all be used. The useful data have, de facto, a bit rate well below that of the demodulation. Also, this solution is very difficult to implement because some of the demodulation operations use loops with low time constants, which requires the availability of the symbols immediately preceding the current symbol. This is not compatible with parallel operation. The many calculations involved in these operations must therefore be performed at a frequency corresponding to the number of operations. This is not always possible because a working frequency of the order of several times the symbol frequency would then be necessary. Thus, for the DFE (Decision Feedback Equalizer, a term well known to those skilled in the art), it is necessary, during a symbol time, to perform a subtraction, make a decision (determine the symbol of the constellation that is closest to the received symbol), and perform a multiplication and at least one addition.